Accelerated plasma clean

ABSTRACT

A method and apparatus that reduces the time required to clean a processing chamber employing a reactive plasma cleaning process. A plasma is formed in an Astron fluorine source generator from a flow of substantially pure inert-source gas. After formation of the plasma, a flow of a fluorine source gas is introduced therein such that the fluorine source flow accelerates at a rate no greater than 1.67 standard cubic centimeters per second 2  (scc/s 2 ). In this fashion, the plasma contains a plurality of radicals and dissociated inert-source gas atoms, defining a cleaning mixture. The ratio of inert-source gas to fluorine source is greater than 1:1.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 10/115,665,filed Apr. 2, 2002, entitled “Accelerated Plasma Clean,” having ShankarN. Chandran, Scott Hendrickson, Gwendolyn D. Jones, Shankar Venkataramanand Ellie Yieh listed as coinventors; which is a division of U.S.application Ser. No. 09/246,036, filed Feb. 4, 1999. The disclosures ofSer. Nos. 10/115,665 and 09/246,036 are herein incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to substrate processing. Specifically, thepresent invention relates to an apparatus and method for cleaning aprocessing chamber in a substrate processing system which reduces thetime required to complete a dry-clean technique by increasing theflow-rate of cleaning gases therein.

One of the primary steps in the fabrication of modern semiconductordevices is the formation of a layer, such as a metal silicide layer liketungsten silicide (WSi_(x)), on a substrate or wafer. As is well known,such a layer can be deposited by chemical vapor deposition (CVD). In aconventional thermal CVD process, reactive gases are supplied to thesubstrate surface where heat-induced chemical reactions take place toform the desired film. In a conventional plasma-enhanced CVD (PECVD)process, a controlled plasma is formed using radio frequency (RF) energyor microwave energy to decompose and/or energize reactive species inreactant gases to produce the desired film.

One problem that arises during such CVD processes is that unwanteddeposition occurs in the processing chamber and leads to potentiallyhigh maintenance costs. With CVD of a desired film on a wafer, undesiredfilm deposition can occur on any hot surface including the heater orprocess kit parts of the apparatus, because the reactive gases candiffuse everywhere in the processing chamber, even within cracks andaround corners. During subsequent wafer depositions, this excess growthon the heater and/or other parts of the apparatus will accelerate untila continuous metal silicide film is grown on the heater and/or theseother parts. Over time, failure to clean the residue from the CVDapparatus often results in degraded, unreliable processes and defectivewafers. When excess deposition starts to interfere with the CVD system'sperformance, the heater and other process kit parts (such as the shadowring and gas distribution faceplate) can be removed and replaced toremove unwanted accumulations in the CVD system. Depending on which andhow many parts need replacing and the frequency of the replacement, thecost of maintaining the substrate processing system can become veryhigh.

In these CVD processes, a reactive plasma cleaning is regularlyperformed to remove the unwanted deposition material from the processingchamber walls, heater, and other process kit parts of the processingchamber. Commonly performed between deposition steps for every wafer orevery n wafers, a reactive plasma cleaning procedure that is performedas a standard processing chamber operation where the etching gas is usedto remove or etch the unwanted deposited material. One reactive plasmacleaning procedure is performed in situ in the processing chamberpromotes excitation and/or disassociation of the reactant gases by theapplication of RF energy with capacitively coupled electrodes disposedin the processing chamber. The plasma creates a highly reactive speciesthat reacts with and etches away the unwanted deposition materialpresent in the processing chamber.

In addition to such in situ plasma cleaning procedures and occurring farless frequently, a second cleaning procedure involves opening theprocessing chamber and physically wiping the entire reactor—includingthe processing chamber walls, exhaust and other areas having accumulatedresidue—with a special cloth and cleaning liquids. This cleaningprocedure is commonly referred to as a wet clean, due to the liquidsemployed. Failure to periodically wet clean a CVD apparatus results inaccumulation of impurities that can migrate onto the wafer and causedevice damage. Thus, properly cleaning a CVD apparatus is important forthe smooth operation of substrate processing, improved device yield andbetter product performance. However, the cleaning procedures reduce theavailability of a system for manufacture due to the down-time requiredto complete the procedures.

As an alternative to in situ plasma cleaning, a remote plasma cleaningprocedure may be employed. To that end, a processing chamber isconnected to a remote microwave plasma system. The remote microwaveplasma cleaning procedure reduces the time required to clean theprocessing chamber. The high breakdown efficiency associated with amicrowave plasma provides a higher etch rate (on the order of about 2μm/min), compared to the etch rate of a capacitive RF plasma.

To further increase the etch rate of unwanted deposition materials,improved reactive plasma generators have been developed which provide anincreased flow of reactive radicals into a processing chamber. One suchreactive plasma generator is sold under the trademark ASTRON by AppliedScience and Technology, Inc. of 35 Cabot Road, Woburn, Mass. 01801-1053.A description of the Astron is located at the following Internet addresshttp://www.astex.com/astron.htm. The Astron is a self-contained atomicfluorine generator which uses a low-field toroidal plasma to dissociatea gas flow introduced into the plasma.

What is needed, however, is a reactive plasma cleaning procedure whichfurther reduces the time required to clean a processing chamber, ascompared to the prior art.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method and apparatus that reduces thetime required to clean a processing chamber employing a reactive plasmacleaning technique. Some embodiments of invention do so by employing anAstron fluorine source generator which is in fluid communication with aprocessing chamber and a supply of fluorine source gas and a supply ofinert source gas. In one embodiment, a plasma is formed in the Astronfrom a flow of substantially pure inert source gas. After formation ofthe plasma, a flow of a fluorine source gas is introduced therein suchthat the fluorine source flow accelerates at a rate no greater than 1.67standard cubic centimeters per second² (scc/s²). In this fashion, theplasma contains a plurality of radicals and dissociated inert-source gasatoms, defining a cleaning mixture. Forming the plasma in the absence ofa fluorine source gas flow overcomes a previously unrecognized problem.Specifically, it was found that flowing a great amount of fluorinesource gas into the ASTRON reactive plasma generator would quench theplasma. As a result, the inert-source gas and fluorine source are flowedinto the Astron to ensure that the ratio of the former to the latter isgreater than 1:1 Specifically, it is believed that the suddendissociation of the fluorine source gas atoms causes a pressure spike.By slowly accelerating a flow of fluorine source gas into the plasmawhile ensuring that the aforementioned ratio is satisfied, this problemis overcome.

Furthermore, a maximum etch rate is achieved by ensuring that the ratioof inert source gas to fluorine source gas is greater than 1:1. To thatend, flow of the fluorine source introduced into the plasma acceleratesuntil reaching a steady rate which is typically not less than 8.33scc/s, and the inert source gas is flowed into the Astron reactiveplasma generator at a first rate, typically not less than 13.33 scc/s.Preferably, however, the first rate and the steady rate are establishedso that the ratio of inert-source gas to fluorine source in the cleaningmixture is approximately 3:2. The cleaning mixture is then flowed fromthe Astron fluorine source generator to the processing chamber where itreacts with undesired contamination present therein. Typically, thefluorine source is selected from a group consisting of NF₃, dilute F₂,CF₄, C2F₆, C₃F₈, SF₆, and ClF₃, and the inert-source gas is argon. It ispreferred, however, that the fluorine source is NF₃.

In another embodiment of the present invention, the acceleration of thefluorine-source gas into the Astron is not critically controlled.Rather, the plasma is formed in the absence of a fluorine-source gas, byflowing the inert source gas into the Astron at a flow rate ofapproximately 16.67 scc/s. After formation of the plasma, the fluorinesource gas is flowed into the Astron at a rate of approximately 13.33scc/s. After allowing stabilization of the plasma for approximately twoto five seconds, the flow rate of both the fluorine source gas and theinert source gas are rapidly increased to 36.67 scc/s and 25.00 scc/s,always ensuring that the ratio of inert-source gas to fluorine sourcegas is greater than 1:1.

These and other embodiments of the present invention, as well as itsadvantages and features, are described in more detail in conjunctionwith the text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are vertical, cross-sectional views of one embodiment ofan exemplary substrate processing apparatus, such as a CVD apparatus,which may be used in accordance with the present invention;

FIGS. 1C and 1D are exploded perspective views of parts of the CVDprocessing chamber depicted in FIG. 1A;

FIG. 1E shows an illustrative block diagram of the hierarchical controlstructure of the system control software, according to a specificembodiment;

FIG. 2 is a simplified plan view of a remote microwave plasma sourcecleaning apparatus in accordance with the present invention;

FIG. 3 is a flowchart illustrating the steps for performing a remotemicrowave plasma cleaning procedure in accordance with the presentinvention;

FIG. 4 is a flowchart illustrating the steps for performing a remotemicrowave plasma cleaning procedure in accordance with anotherembodiment of the present invention;

FIG. 5 is a simplified plan view of a hazardous air pollutants (HAPS)measurement system connected to the substrate processing apparatus shownin FIGS. 1A and 1B;

FIG. 6 is a graph showing the make-up of the output gases from asubstrate processing apparatus in accordance with the present invention;and

FIG. 7 is a graph showing the make-up of the output gases from a priorart substrate processing apparatus.

DETAILED DESCRIPTION OF THE INVENTION

I. Exemplary CVD System

Specific embodiments of the present invention may be used with orretrofitted onto a variety of chemical vapor deposition (CVD) or othertypes of substrate processing apparatus. One suitable substrateprocessing apparatus with which the present invention can be used orretrofitted is shown in FIGS. 1A and 1B, which are vertical,cross-sectional views of a CVD system 10, having a vacuum or processingchamber 15 that includes a processing chamber wall 15 a and processingchamber lid assembly 15 b. Processing chamber wall 15 a and processingchamber lid assembly 15 b are shown in exploded, perspective views inFIGS. 1C and 1D.

Referring to FIGS. 1A, 1B, 1C and 1D, CVD system 10 contains a gasdistribution manifold 11 for dispersing process gases to a substrate(not shown) that rests on a resistively-heated pedestal 12 centeredwithin the processing chamber. During processing, the substrate (e.g., asemiconductor wafer) is positioned on a flat (or slightly convex)surface 12 a of pedestal 12. Preferably having a surface of ceramic suchas aluminum nitride, pedestal 12 can be moved controllably between alower loading/off-loading position (depicted in FIG. 1A) and an upperprocessing position (indicated by dashed line 14 in FIG. 1A and shown inFIG. 1B), which is closely adjacent to manifold 11. A centerboard (notshown) includes sensors for providing information on the position of thewafers.

Deposition and carrier gases are introduced into processing chamber 15through perforated holes 13 b (FIG. 1D) of a conventional flat, circulargas distribution face plate 13 a. More specifically, deposition processgases flow (indicated by arrow 40 in FIG. 1B) into the processingchamber through the inlet manifold 11, through a conventional perforatedblocker plate 42 and then through holes 13 b in gas distributionfaceplate 13 a.

Before reaching the manifold, deposition and carrier gases are inputfrom gas sources 7 through gas supply lines 8 (FIG. 1B) into a gasmixing block or system 9 where they are combined and then sent tomanifold 11. It is also possible, and desirable in some instances, todirect deposition and carrier gases directly from supply lines 8 tomanifold 11. In such a case, gas mixing system 9 is bypassed. In othersituations, any of gas lines 8 may bypass gas mixing system 9 andintroduce gases through passages (not shown) in the bottom of processingchamber 15. As shown in FIG. 1B, there are three gas supply lines 8 in aspecific embodiment to deposit, for example, WSiX_(x-) a first line 8 asupplies a silicon-containing gas (e.g., dichlorosilane (SiH₂Cl₂)referred to as “DCS” from a DCS source from gas source 7 a) into gasmixing system 9, while a second line 8 b supplies a tungsten-containinggas (e.g., tungsten hexafluoride (WF₆) from a WF₆ source from gas source7 b) into gas mixing system 9. For each line 8 a and 8 b, a carrier gas(e.g., argon from argon sources in gas sources 7 a and 7 b) can besupplied with the process to stabilize gas flows as appropriate and toeven the gas flow between the two lines into mixing system 9. Suchmixing of gases (DCS and WF₆) upstream of processing chamber 15 isbelieved to result in more uniform gas distribution into the processingchamber, thereby resulting in greater uniformity in the depositedWSi_(x) film. A third supply line 8 c introduces an inert-source purgegas (e.g., argon from a gas source 7 c) from the bottom of theprocessing chamber to keep deposition gases away from the area of theprocessing chamber below pedestal 12. In some preferred embodiments, anadditional silicon source (e.g., silane (SiH₄) from source 7 a may besupplied to gas line 8 a.

Generally, the supply line for each process gas includes (i) severalsafety shut-off valves (not shown) that can be used to automatically ormanually shut off the flow of process gas into the processing chamber,and (ii) mass flow controllers (MFCs) (also not shown) that measure theflow of gas through the supply line. When toxic gases are used in theprocess, the several safety shut-off valves are positioned on each gassupply line in conventional configurations.

The deposition process performed in the CVD system 10 can be either athermal process or a plasma-enhanced process. In a plasma-enhancedprocess, an RF power supply 44 applies electrical power between the gasdistribution faceplate 13 a and pedestal 12 to excite the process gasmixture to form a plasma within the cylindrical region between thefaceplate 13 a and pedestal 12. (This region will be referred to hereinas the “reaction region”). Constituents of the plasma react to deposit adesired film on the surface of the semiconductor wafer supported onpedestal 12. RF power supply 44 can be a mixed frequency RF power supplythat typically supplies power at a high RF frequency (RF1) of 13.56Megahertz (MHz) and at a low RF frequency (RF2) of 360 kilohertz (kHz)to enhance the decomposition of reactive species introduced into theprocessing chamber 15. Of course, RF power supply 44 can supply eithersingle- or mixed-frequency RF power (or other desired variations) tomanifold 11 to enhance the decomposition of reactive species introducedinto processing chamber 15. In a thermal process, RF power supply 44 isnot utilized, and the process gas mixture thermally reacts to depositthe desired film on the surface of the semiconductor wafer supported onpedestal 12, which is resistively heated to provide the thermal energyneeded for the reaction.

During a plasma-enhanced deposition process, the plasma heats the entireCVD system, including the walls of the processing chamber body 15 asurrounding the exhaust passageway 23 and the shut-off valve 24. Duringa thermal deposition process, heated pedestal 12 causes heating of CVDsystem. When the plasma is not turned on, or during a thermal depositionprocess, a hot liquid is circulated through the walls 15a of CVD systemto maintain the processing chamber at an elevated temperature. Fluidsused to heat the processing chamber walls 15a include the typical fluidtypes, i.e., water-based ethylene glycol or oil-based thermal transferfluids. This heating beneficially reduces or eliminates condensation ofundesirable reactant products and improves the elimination of volatileproducts of the process gases and contaminants that might otherwisecondense on the walls of cool vacuum passages and migrate back into theprocessing chamber during periods of no gas flow.

The remainder of the gas mixture that is not deposited in a layer,including reaction products, is evacuated from the processing chamber bya vacuum pump (not shown). Specifically, the gases are exhausted throughan annular slot-shaped orifice 16 surrounding the reaction region andinto an annular exhaust plenum 17. The annular slot 16 and the plenum 17are defined by the gap between the top of the processing chamber'scylindrical side wall 15 a (including the upper dielectric lining 19 onthe wall) and the bottom of the circular processing chamber lid 20. The360° circular symmetry and uniformity of the slot orifice 16 and theplenum 17 are important to achieving a uniform flow of process gasesover the wafer to deposit a uniform film on the wafer.

The gases flow underneath a lateral extension portion 21 of the exhaustplenum 17, past a viewing port (not shown), through a downward-extendinggas passage 23, past a vacuum shut-off valve 24 (whose body isintegrated with the lower processing chamber wall 15 a), and into theexhaust outlet 25 that connects to the external vacuum pump (not shown)through a foreline (also not shown).

The wafer support platter of resistively-heated pedestal 12 is heatedusing an embedded single-loop heater element configured to make two fullturns in the form of parallel concentric circles. An outer portion ofthe heater element runs adjacent to a perimeter of the support platter,while an inner portion runs on the path of a concentric circle having asmaller radius. The wiring to the heater element passes through the stemof pedestal 12. Pedestal 12 may be made of material including aluminum,ceramic, or some combination thereof.

Typically, any or all of the processing chamber lining, gas inletmanifold faceplate, and various other reactor hardware are made out ofmaterial such as aluminum, anodized aluminum, or ceramic. An example ofsuch CVD apparatus is described in commonly assigned U.S. Pat. No.5,558,717 entitled “CVD Processing Chamber,” issued to Zhao et al.,hereby incorporated by reference in its entirety.

A lift mechanism and motor 32 (FIG. 1A) raises and lowers the heaterpedestal assembly 12 and its wafer lift pins 12b as wafers aretransferred by a robot blade (not shown) into and out of the body of theprocessing chamber through an insertion/removal opening 26 in the sideof the processing chamber 10. The motor 32 raises and lowers pedestal 12between a processing position 14 and a lower wafer-loading position. Themotor, valves or flow controllers connected to the supply lines 8, gasdelivery system, throttle valve, RF power supply 44, processing chamberand substrate heating systems are all controlled by a system controller34 (FIG. 1B) over control lines 36, of which only some are shown.Controller 34 relies on feedback from optical sensors to determine theposition of movable mechanical assemblies such as the throttle valve andpedestal which are moved by appropriate motors controlled by controller34.

In a preferred embodiment, the system controller includes a hard diskdrive (memory 38), a floppy disk drive and a processor 37. The processorcontains a single-board computer (SBC), analog and digital input/outputboards, interface boards and stepper motor controller boards. Variousparts of CVD system 10 conform to the Versa Modular European (VME)standard which defines board, card cage, and connector dimensions andtypes. The VME standard also defines the bus structure as having a16-bit data bus and a 24-bit address bus.

System controller 34 controls all of the activities of the CVD machine.The system controller executes system control software, which is acomputer program stored in a computer-readable medium such as a memory38. Preferably, memory 38 is a hard disk drive, but memory 38 may alsobe other kinds of memory. The computer program includes sets ofinstructions that dictate the timing, mixture of gases, chamberpressure, chamber temperature, RF power levels, pedestal position, andother parameters of a particular process. Other computer programs storedon other memory devices including, for example, a floppy disk or otheranother appropriate drive, may also be used to operate controller 34.

The interface between a user and controller 34 is via a CRT monitor andlight pen. In the preferred embodiment two monitors are used, onemounted in the clean room wall for the operators and the other, behindthe wall for the service technicians. The monitors simultaneouslydisplay the same information, but only one light pen is enabled. A lightsensor in the tip of the light pen detects light emitted by CRT display.To select a particular screen or function, the operator touches adesignated area of the display screen and pushes the button on the pen.The touched area changes its highlighted color, or a new menu or screenis displayed, confirming communication between the light pen and thedisplay screen. Other devices, such as a keyboard, mouse, or otherpointing or communication device, may be used instead of or in additionto the light pen to allow the user to communicate with controller 34.

The process for depositing the film can be implemented using a computerprogram product that includes computer code to be executed by controller34. The computer code can be written in any conventional computerreadable programming language: for example, 68000 assembly language, C,C++, Pascal, Fortran or others. Suitable program code is entered into asingle file, or multiple files, using a conventional text editor and isstored or embodied in a computer-usable medium, such as a memory systemof the computer. If the entered computer code is associated with a highlevel language, the computer code is compiled, and the resultantcompiler code is then linked with an object code of precompiled Windows™library routines. To execute the linked, compiled object code, thesystem user invokes the object code causing the computer system to loadthe code in memory. The CPU then reads and executes the code to performthe tasks identified in the program.

FIG. 1E is an illustrative block diagram of the hierarchical controlstructure of the system control software, computer program 70, accordingto a specific embodiment. Using the light pen interface, a user enters aprocess set number and processing chamber numbers into a processselector subroutine 73 in response to menus or screens displayed on theCRT monitor. The process sets are predetermined sets of processparameters necessary to carry out specified processes, and areidentified by predefined set numbers. The process selector subroutine 73identifies (i) the desired processing chamber and (ii) the desired setof process parameters needed to operate the processing chamber forperforming the desired process. The process parameters for performing aspecific process relate to process conditions such as process gascomposition and flow rates, temperature, pressure, plasma conditionssuch as microwave power levels or RF power levels and the low frequencyRF frequency, cooling gas pressure, and processing chamber walltemperature. These parameters are provided to the user in the form of arecipe and are entered utilizing the light pen/CRT monitor interface.

The signals for monitoring the process are provided by the analog anddigital input boards of the system controller, and the signals forcontrolling the process are output on the analog and digital outputboards of CVD system 10.

A process sequencer subroutine 75 comprises program code for acceptingthe identified processing chamber and set of process parameters from theprocess selector subroutine 73 and for controlling operation of thevarious processing chambers. Multiple users can enter process setnumbers and processing chamber numbers, or a single user can entermultiple process set numbers and processing chamber numbers, so thesequencer subroutine 75 operates to schedule the selected processes inthe desired sequence. Preferably, the sequencer subroutine 75 includes aprogram code to perform the steps of (i) monitoring the operation of theprocessing chambers to determine if the processing chambers are beingused, (ii) determining what processes are being carried out in theprocessing chambers being used, and (iii) executing the desired processbased on availability of a processing chamber and the type of process tobe carried out. Conventional methods of monitoring the processingchambers can be used, such as polling. When scheduling which process isto be executed, sequencer subroutine 75 takes into consideration thepresent condition of the processing chamber being used in comparisonwith the desired process conditions for a selected process, or the “age”of each particular user-entered request, or any other relevant factor asystem programmer desires to include for determining schedulingpriorities.

Once the sequencer subroutine 75 determines which processing chamber andprocess set combination is going to be executed next, the sequencersubroutine 75 initiates execution of the process set by passing theparticular process set parameters to a processing chamber managersubroutine 77 a-c, which controls multiple processing tasks in aprocessing chamber 15 according to the process set determined by thesequencer subroutine 75. For example, the processing chamber managersubroutine 77 a comprises program code for controlling sputtering andCVD process operations in the processing chamber 15. The processingchamber manager subroutine 77 also controls execution of variousprocessing chamber component subroutines that control operation of theprocessing chamber components necessary to carry out the selectedprocess set. Examples of processing chamber component subroutines aresubstrate positioning subroutine 80, process gas control subroutine 83,pressure control subroutine 85, heater control subroutine 87, and plasmacontrol subroutine 90. Those having ordinary skill in the art willreadily recognize that other processing chamber control subroutines canbe included depending on what processes are to be performed in theprocessing chamber 15. In operation, the processing chamber managersubroutine 77 a selectively schedules or calls the process componentsubroutines in accordance with the particular process set beingexecuted. The processing chamber manager subroutine 77a schedules theprocess component subroutines much like the sequencer subroutine 75schedules which processing chamber 15 and process set are to be executednext. Typically, the processing chamber manager subroutine 77 a includessteps of monitoring the various processing chamber components,determining which components need to be operated based on the processparameters for the process set to be executed, and causing execution ofa processing chamber component subroutine responsive to the monitoringand determining steps.

Operation of particular processing chamber component subroutines willnow be described with reference to FIG. 1E. The substrate positioningsubroutine 80 comprises program code for controlling processing chambercomponents that are used to load the substrate onto pedestal 12 and,optionally, to lift the substrate to a desired height in the processingchamber 15 to control the spacing between the substrate and the gasdistribution manifold 11. When a substrate is loaded into the processingchamber 15, pedestal 12 is lowered to receive the substrate, andthereafter, pedestal 12 is raised to the desired height in theprocessing chamber to maintain the substrate at a first distance orspacing from the gas distribution manifold during the CVD process. Inoperation, the substrate positioning subroutine 80 controls movement ofpedestal 12 in response to process set parameters related to the supportheight that are transferred from the processing chamber managersubroutine 77 a.

The process gas control subroutine 83 has program code for controllingprocess gas composition and flow rates. The process gas controlsubroutine 83 controls the open/close position of the safety shut-offvalves, and also ramps the mass flow controllers up or down to obtainthe desired gas flow rate. The process gas control subroutine 83 isinvoked by the processing chamber manager subroutine 77 a, as are allprocessing chamber component subroutines, and receives from theprocessing chamber manager subroutine process parameters related to thedesired gas flow rates. Typically, the process gas control subroutine 83operates by opening the gas supply lines and repeatedly (i) reading therequisite mass flow controllers, (ii) comparing the readings to thedesired flow rates received from the processing chamber managersubroutine 77 a, and (iii) adjusting the flow rates of the gas supplylines as necessary. Furthermore, the process gas control subroutine 83includes steps for monitoring the gas flow rates for unsafe rates andfor activating the safety shut-off valves when an unsafe condition isdetected.

In some processes, an inert-source gas such as helium or argon is flowedinto the processing chamber 15 to stabilize the pressure in theprocessing chamber before reactive process gases are introduced. Forthese processes, the process gas control subroutine 83 is programmed toinclude steps for flowing the inert-source gas into the processingchamber 15 for an amount of time necessary to stabilize the pressure inthe processing chamber, and then the steps described above are carriedout. Additionally, if a process gas is to be vaporized from a liquidprecursor, for example, tetraethylorthosilicate (“TEOS”), the processgas control subroutine 83 is written to include steps for bubbling adelivery gas, such as helium, through the liquid precursor in a bubblerassembly or introducing a carrier gas, such as helium or nitrogen, to aliquid injection system. When a bubbler is used for this type ofprocess, the process gas control subroutine 83 regulates the flow of thedelivery gas, the pressure in the bubbler, and the bubbler temperaturein order to obtain the desired process gas flow rates. As discussedabove, the desired process gas flow rates are transferred to the processgas control subroutine 83 as process parameters. Furthermore, theprocess gas control subroutine 83 includes steps for obtaining thenecessary delivery gas flow rate, bubbler pressure, and bubblertemperature for the desired process gas flow rate by accessing a storedtable containing the required values for a given process gas flow rate.Once the required values are obtained, the delivery gas flow rate,bubbler pressure and bubbler temperature are monitored, compared to therequired values and adjusted accordingly.

The pressure control subroutine 85 comprises program code forcontrolling the pressure in the processing chamber 15 by regulating thesize of the opening of the throttle valve in the exhaust system of theprocessing chamber. The size of the opening of the throttle valve is setto control the processing chamber pressure to the desired level inrelation to the total process gas flow, size of the processing chamber,and pumping set-point pressure for the exhaust system. When the pressurecontrol subroutine 85 is invoked, the target pressure level is receivedas a parameter from the processing chamber manager subroutine 77 a. Thepressure control subroutine 85 measures the pressure in the processingchamber 15 by reading one or more conventional pressure manometersconnected to the processing chamber, to compare the measured value(s) tothe target pressure, to obtain PID (proportional, integral, anddifferential) values from a stored pressure table corresponding to thetarget pressure, and to adjust the throttle valve according to the PIDvalues obtained from the pressure table. Alternatively, the pressurecontrol subroutine 85 can be written to open or close the throttle valveto a particular opening size to regulate the processing chamber 15 tothe desired pressure.

The heater control subroutine 87 comprises program code for controllingthe current to a heating unit that is used to heat the substrate 20. Theheater control subroutine 87 is also invoked by the processing chambermanager subroutine 77 a and receives a target, or set-point, temperatureparameter. The heater control subroutine 87 measures the temperature bymeasuring voltage output of a thermocouple located in a pedestal 12,comparing the measured temperature to the set-point temperature, andincreasing or decreasing current applied to the heating unit to obtainthe set-point temperature. The temperature is obtained from the measuredvoltage by looking up the corresponding temperature in a storedconversion table or by calculating the temperature using a fourth-orderpolynomial. When an embedded loop is used to heat pedestal 12, theheater control subroutine 87 gradually controls a ramp up/down ofcurrent applied to the loop. Additionally, a built-in fail-safe mode canbe included to detect process safety compliance and can shut downoperation of the heating unit if the processing chamber 15 is notproperly set up.

The plasma control subroutine 90 comprises program code for setting thelow and high frequency RF power levels applied to the process electrodesin the processing chamber 15 and for setting the low frequency RFfrequency employed. Plasma control subroutine 90 also includes programcode for turning on and setting/adjusting the power levels applied tothe magnetron or other microwave source used in the present invention.Similarly to the previously described processing chamber componentsubroutines, the plasma control subroutine 90 is invoked by theprocessing chamber manager subroutine 77 a.

The above CVD system description is mainly for illustrative purposes,and other equipment such as electron cyclotron resonance (ECR) plasmaCVD devices, induction coupled RF high density plasma CVD devices, orthe like may be used with the present invention to provide upgradedapparatus. Additionally, variations of the above-described system, suchas variations in pedestal design, heater design, RF power frequencies,location of RF power connections and others are possible. For example,the wafer could be supported and heated by quartz lamps. It should berecognized that the present invention is not necessarily limited to usewith or retrofitting of any specific apparatus.

II. Remote Plasma Cleaning Source

As discussed above, unwanted deposition may occur on any hot surface inthe processing chamber 15, including the heater, process kit parts ofthe apparatus and the processing chamber walls. To remove the unwantedresidue, a remote plasma source 300, shown in FIG. 2, is in fluidcommunication with the processing chamber 15, a supply 304 of a fluorinesource gas, such as nitrogen tri-fluoride (NF₃), and a supply of ainert-source gas such as argon (Ar) or helium (He₂). In this fashion,the fluorine source gas from supply 304 and the inert-source gas fromsupply 306 may be flowed, under vacuum from the processing chamber'spumping and exhaust system (not shown) into the plasma source 300 wherea plasma is formed therefrom. The plasma comprises a plurality ofradicals formed from the fluorine source gas and dissociated atoms fromthe inert-source gas. The aforementioned mixture of radicals anddissociated atoms defines a cleaning mixture. The cleaning mixtureflows, under force of vacuum, from the plasma source to the processingchamber 15. Upon entering the processing chamber 15, the fluorineradicals in the cleaning mixture react with the unwanted depositionpresent in the processing chamber 15. For example, were tungstensilicide present in the processing chamber 15, and the processingchamber 15 and chamber components were manufactured from aluminum, thefluorine radicals would form WF₆, SiF₄ and AlF₃.

In a preferred embodiment, the plasma source 300 is a self-containedatomic fluorine generator which uses a low-field toroidal plasma todissociate a gas flow introduced into the plasma and is sold by AppliedScience and Technology, Inc. of 35 Cabot Road, Woburn, Mass. 01801-1053under the trademark Astron. The Astron provides a very high flow rate offluorine radicals into the processing chamber, compared to competingfluorine generators. The flow rate of the fluorine radicals isproportional to the etch rate of unwanted deposition in the processingchamber 15. Therefore, employing the Astron to perform a reactive plasmacleaning procedure substantially reduces the time required to clean theprocessing chamber 15.

A problem was encountered with the Astron, however, in that rapidlyincreasing the volume of a fluorine source gas in the Astron quenchedthe plasma; thus, limiting the time reduction afforded by the Astron. Asolution to the aforementioned problem is based upon the discovery thatthe quenching of the plasma was due to a pressure spike that resultedfrom flowing the fluorine source gas into the Astron. The pressure spikeis attributed to the dissociation of the multi-atom fluorine source intoa great number of radicals that are present, for a fraction of a second,upon initiation of the fluorine source into the plasma. It was foundthat by providing a ratio of the inert-source gas to the fluorine sourcegas present in the Astron in excess of 1:1, the aforementioned pressurespike is avoided and the plasma is not quenched. To that end, it ispreferred that the plasma is formed in the Astron in the absence of aflow of fluorine source gas, i.e., only a flow of inert-source gas ispresent in the Astron when the plasma is initially formed. Thereafter, aflow of the fluorine source gas is introduced into the Astron and isaccelerated at a rate of 1.67 scc/s², while ensuring that theaforementioned ratio is satisfied. Preferably the flow of fluorinesource gas is accelerated until reaching a steady rate, with the rate ofthe argon and the steady rate of the fluorine source gas beingestablished so that the ratio of inert-source gas to fluorine source gasis 3:2.

Referring to FIG. 3 in an exemplary method of operating the Astron, theAstron is isolated from supply 304 at step 402. At step 404, argon isflowed into the Astron at a rate of 13.33 scc/s. At step 406, a plasmais formed. At step 408 the fluorine source gas is flowed into the Astronso that, in the first second of flow, the rate of the fluorine sourcegas is 1.67 scc/s and is accelerated at constant rate of 1.67 scc/s²until reaching a steady rate of 8.33 scc/s at which time theacceleration of the flow is terminated. Thereafter, at step 410 the flowrate of both the inert-source gas and the fluorine source gas into theAstron are increased to approximately 25 scc/s and 15.83 scc/s,respectively. The plasma consisting of 25 scc/s flow rate ofinert-source gas and 15.83 scc/s flow rate of fluorine source gasdefines a cleaning mixture. At step 412, the cleaning mixture isintroduced into the processing chamber 15.

Referring to FIG. 4, in another exemplary method according to thepresent invention, the Astron is isolated from supply 304 at step 502.At step 504, argon is flowed into the Astron at a rate of 16.67 scc/s.At step 506, a plasma is formed. At step 508 the fluorine source gas isflowed into the Astron at a rate of 13.33 scc/s, and after two to fiveseconds of plasma stabilization the flows of both the fluorine sourcegas and the inert source gas are rapidly increased, at step 510.Specifically, the fluorine source gas is flowed so that the rateincreases to 25.00 scc/s, and the inert source gas is flows so that therate increases to 36.67 scc/s. However, the ratio of inert-source gas tofluorine source gas is greater than 1:1.

An additional benefit of the present invention concerns the use of argonas the inert-source gas source. Specifically, it was found that byemploying the present invention, hazardous air-pollutant sources (HAPS),such as fluorine, are reduced. To determine the quantity of HAPSproduced during processing, measurements are made of gases exiting theCVD system 10 via the exhaust passageway 23, shown in FIG. 1A byconnecting a HAPS measurement system 100 thereto.

As shown in FIG. 5, the HAPS measurement system 100 includes a samplingcylinder 102, one end of which is connected to the exhaust passageway23, with the remaining end connected to a building exhaust system (notshown). The sampling cylinder 102 includes two spaced-apart samplingports 104 a and 104 b, with a UTI QualiTrace quadrupole massspectrometer (QMS) system 106 is in fluid communication with thesampling cylinder 102. An abatement system 108 is also in fluidcommunication with the sampling cylinder 102 and the exhaust passageway23. The QMS system 106 operates in either a “static” mode to collect acomplete spectrum over the entire 200 amu mass range, or a “dynamic”mode to monitor several different ions continuously as a function oftime. The instrument is calibrated with a dynamic dilution system toaccurately measure gas standards right before each emission measurementto ensure precise quantification of the effluents over a concentrationrange spanning over five orders of magnitude. The exhaust gas sample iscontinuously drawn through the exhaust passageway 23 into the QMS system106 via a pump 110. The pressure in the QMS system 106 is maintainedbetween 650 and 700 Torr via a throttle valve 112. Briefly, NF₃ flowsthrough the remote plasma source 300 and decomposes into F and N atoms.The highly reactive F atoms then react with deposition residue, mainlySiO₂ to form gaseous effluents, SiF₄ and O₂.

The second method of investigation utilizes the Fourier transformedinfrared (FTIR) absorption spectroscopy to identify the effluent speciesfrom the processing chamber 15, qualitatively. The FTIR collects asingle infrared interferogram of a static sample instead of monitoring agas stream continuously over time. After a comparison with the knownspectrum of certain gases, the effluent species could be positivelyidentified. This is a helpful for HF measurement, since the HF ion has asimilar mass-to-charge ratio as the doubly charged Ar ion. Therefore,FTIR is mainly used as a supplement to the QMS, which has the bestresolution and sensitivity.

Results of the aforementioned measurements are shown in FIGS. 6 and 7.FIG. 6 shows an RGA analysis of gases produced by the present inventionand FIG. 7 shows an RGA analysis employing a prior art remote plasmasource generator in place of the Astron fluorinator using the systemdescribed above. Based on-the aforementioned measurements, it isrealized that by employing the Astron fluorinator, the HAPS produced arereduced by 50%.

Although the fluorine source gas is described as being NF₃, any one of avariety of fluorine sources may be employed such as dilute F₂, CF₄,C₂F₆, C₃F₈, SF₆, and ClF₃. The scope of the invention should, therefore,be determined not with reference to the above description, but shouldinstead be determined with reference to the appended claims, along withthe full scope of equivalents to which such claims are entitled.

1. A method of removing residue from a substrate processing chamber, themethod comprising: initiating a plasma within the remote plasma chamberwhile flowing a first gas consisting of a substantially pure inert gasinto a remote plasma chamber; flowing a nitrogen trifluoride gas intothe remote plasma chamber and into the plasma; creating a plurality ofreactive fluorine species in the remote plasma chamber while increasinga rate at which the nitrogen trifluoride gas is introduced into theremote plasma chamber; and introducing the plurality of reactivefluorine species into the substrate processing chamber; wherein the flowrates of the first gas and nitrogen trifluoride gas introduced into theremote plasma chamber during the method of removing residue arecontrolled to avoid an abrupt change in pressure within the remoteplasma chamber that extinguishes the plasma and wherein a ratio of inertgas to nitrogen trifluoride introduced into the remote plasma chamber ismaintained above 1:1 while the rate at which nitrogen trifluoride isintroduced is increased.
 2. The method of claim 1 wherein the first gasconsists of argon.
 3. The method of claim 1 wherein the remote plasmagenerator uses a low-field toroidal to generate the plasma.
 4. Themethod of claim 1 wherein the rate at which the nitrogen trifluoride gasis introduced into the remote plasma chamber is accelerated at aconstant rate until reaching a steady state rate and then increased fromthe steady state rate to a second rate.
 5. The method of claim 4 whereinthe plurality of reactive fluorine-species are not introduced into thesubstrate processing chamber until after rate of the nitrogentrifluoride gas reaches the second rate.
 6. The method of claim 1wherein the rate at which the nitrogen trifluoride gas is introducedinto the remote plasma chamber is increased from a first rate to asecond rate and wherein the plurality of reactive fluorine-species arenot introduced into the substrate processing chamber until after therate of the nitrogen trifluoride gas reaches the second rate.
 7. Amethod of removing residue from a substrate processing chamber, themethod comprising: providing a remote plasma chamber fluidly coupled tothe substrate processing chamber; initiating a plasma in the remoteplasma chamber from a flow of a substantially pure inert gas; formingreactive radicals in the remote plasma chamber from a clean gasintroduced into the remote plasma chamber comprising afluorine-containing gas and an inert gas; and transporting the reactiveradicals from the remote plasma chamber to the substrate processingchamber while increasing a rate at which the fluorine-containing gas isintroduced into the remote plasma chamber; wherein the flow rate ofgases introduced into the remote plasma chamber during the method ofremoving residue is controlled to avoid an abrupt change in pressurewithin the remote plasma chamber that extinguishes the plasma andwherein a ratio of inert gas to fluorine-containing gas in the clean gasis maintained above 1:1 while the rate at which the fluorine-containinggas is introduced into the remote plasma chamber is increased.
 8. Themethod of claim 7 wherein the fluorine-containing gas comprises NF₃. 9.The method of claim 7 wherein the fluorine-containing gas comprises NF₃.10. The method of claim 7 wherein the remote plasma generator uses alow-field toroidal to generate the plasma.
 11. The method of claim 7wherein the rate at which the fluorine-containing gas is introduced intothe remote plasma chamber is accelerated at a constant rate untilreaching a steady state rate and then increased from the steady staterate to a second rate.
 12. The method of claim 11 wherein the pluralityof reactive fluorine-species are not introduced into the substrateprocessing chamber until after rate of the fluorine-containing gasreaches the second rate.
 13. The method of claim 7 wherein the rate atwhich the fluorine-containing gas is introduced into the remote plasmachamber is increased from a first rate to a second rate and wherein theplurality of reactive fluorine-species are not introduced into thesubstrate processing chamber until after rate of the fluorine-containinggas reaches the second rate.